One major advantage of the free piston Stirling engine is that the working gas can be entirely sealed within the engine to prevent its contamination or loss by leakage. It is undesirble to lubricate the pistons of the free piston Stirling engine with traditional lubricants, such as petroleum based oil and grease, because such lubricants vaporize into the working gas and reduce its efficiency.
Nonetheless, it is still desirable to lubricate such engines for the purpose of extending the life of the engine and reducing its wear and maintenance.
It is therefore an object of the present invention to effect the hydrodynamic lubrication of pistons through use of the fluids which act upon or are acted upon by the pistons in the operation of the device, and particularly to lubricate the pistons of Stirling engines with the working gas of the engine.